An Ongoing Tidal Capture in the Large Magellanic Cloud: The Low-mass Star Cluster KMK 88-10 Captured by the Massive Globular Cluster NGC 1835?*
Camilla Giusti,
Mario Cadelano,
Francesco R. Ferraro
et al.
Abstract:In the context of a project aimed at characterizing the dynamical evolution of old globular clusters in the Large Magellanic Cloud, we have secured deep HST/WFC3 images of the massive cluster NGC 1835. In the field of view of the acquired images, at a projected angular separation of approximately 2′ from the cluster, we detected the small stellar system KMK 88-10. The observations provided the deepest color–magnitude diagram ever obtained for this cluster, revealing that it hosts a young stellar population wit… Show more
We present a high-angular-resolution multi-wavelength study of
the massive globular cluster NGC 1835 in the Large Magellanic
Cloud. Thanks to a combination of optical and near-ultraviolet
images acquired with the WFC3 on board the HST, we performed a
detailed inspection of the stellar population in this stellar
system, adopting a `UV-guided search' to
optimize the detection of relatively hot stars. This allowed us to discover a remarkably extended horizontal branch (HB): it spans more than 4.5 magnitudes in both the optical and the near-ultraviolet bands, and its colour (temperature) ranges from the region redder than the instability strip up to effective temperatures of 30,000 K. This is the first time that such a feature has been detected in an
extragalactic cluster, demonstrating that the physical conditions
responsible for the formation of extended HBs are ubiquitous. The HB of NGC 1835
includes a remarkably large population of RR Lyrae (67 confirmed variables and 52 new candidates). The acquired dataset was also used to redetermine the cluster
distance modulus, reddening, and absolute age:
$(m-M)_0=18.58$, $E(B-V)=0.08$, and $t=12.5$ Gyr.
We present a high-angular-resolution multi-wavelength study of
the massive globular cluster NGC 1835 in the Large Magellanic
Cloud. Thanks to a combination of optical and near-ultraviolet
images acquired with the WFC3 on board the HST, we performed a
detailed inspection of the stellar population in this stellar
system, adopting a `UV-guided search' to
optimize the detection of relatively hot stars. This allowed us to discover a remarkably extended horizontal branch (HB): it spans more than 4.5 magnitudes in both the optical and the near-ultraviolet bands, and its colour (temperature) ranges from the region redder than the instability strip up to effective temperatures of 30,000 K. This is the first time that such a feature has been detected in an
extragalactic cluster, demonstrating that the physical conditions
responsible for the formation of extended HBs are ubiquitous. The HB of NGC 1835
includes a remarkably large population of RR Lyrae (67 confirmed variables and 52 new candidates). The acquired dataset was also used to redetermine the cluster
distance modulus, reddening, and absolute age:
$(m-M)_0=18.58$, $E(B-V)=0.08$, and $t=12.5$ Gyr.
In the context of the study of the size--age relationship observed in star clusters in the Large Magellanic Cloud (LMC) and the investigation of its origin, we present the determination of the structural parameters and
the dynamical age of the massive cluster NGC 1835. We used the
powerful combination of optical and near-ultraviolet images acquired
with the WFC3 on board the HST to construct the star density profile
from resolved star counts, determining the values of the core,
half-mass, and tidal radii through comparison with the King model
family. The same data also allowed us to evaluate the dynamical age
of the cluster by using the `dynamical clock'. This is an
empirical method that quantifies the level of the central segregation of
blue stragglers stars (BSSs) within the cluster half-mass radius by
means of the $A^+_ rh $ parameter, which is defined as the area
enclosed between the cumulative radial distribution of BSSs and that
of a reference (lighter) population. The results confirm that NGC
1835 is a very compact cluster with a core radius of only 0.84
pc. The estimated value of $A^+_ rh 0.04$) is the largest measured so far in the LMC clusters, providing evidence of a highly dynamically evolved stellar system. NGC 1835 fits nicely into the correlation between rh $ and the central relaxation
time and in the anti-correlation between $A^+_ rh $ and the core
radius defined by the Galactic and Magellanic Cloud clusters investigated to date.
The presence of young stellar populations in the Large Magellanic Cloud cluster NGC 1783 has caught significant attention, with suggestions ranging from it being a genuine secondary stellar generation to a population of blue straggler stars or simply contamination from background stars. Thanks to multiepoch observations with the Hubble Space Telescope, proper motions for stars within the field of NGC 1783 have been derived, thus allowing accurate cluster membership determination. Here, we report that the younger stars within NGC 1783 indeed belong to the cluster and their spatial distribution is more extended compared to the bulk of the older stellar population, consistent with previous studies. Through N-body simulations, we demonstrate that the observed characteristics of the younger stars cannot be explained solely by blue straggler stars in the context of the isolated dynamical evolution of NGC 1783. Instead, accretion of the external, low-mass stellar system can better account for both the inverse spatial concentration and the radial velocity isotropy of the younger stars. We propose that NGC 1783 may have accreted external stars from low-mass stellar systems, resulting in a mixture of external younger stars and blue straggler stars from the older bulk population, thereby accounting for the characteristics of the younger sequence.
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